This invention relates to novel biphenyl esters, to liquid crystalline mixtures comprising them, and to processes for their preparation and their use as components in such mixtures, which are useful as dielectrics for liquid crystal display elements.
To an increasing extent the changes of the optical properties, such as light scattering, birefringence, reflecting power or color, of nematic or nematic-cholesteric liquid crystalline materials under the influence of an electric field are employed for converting electrical voltages pulses into optical signals. The function of such liquid crystal display elements thereby depends, for example, upon the phenomenon of dynamic scattering, the deformation of aligned phases or the Schadt-Helfrich effect in the twisted cell.
For the technical application of these effects in electronic constructional elements, liquid crystalline materials are required which satisfy a plurality of requirements. Especially important are chemical stability towards moisture, air and physical influences, such as heat, radiation in the infrared, visible and untra-violet range and electrical direct and alternating fields. Furthermore, liquid crystalline materials, to be useable, should have a liquid crystal mesophase in the temperature range of at least +10.degree. C. to +60.degree. C., preferably 0.degree. C. to 60.degree. C., and a viscosity at room temperature of not more than 70 cP. Finally, they must not exhibit any inherent absorption in the range of visible light, i.e., they must be colorless.
Colorless liquid crystalline compounds are known which satisfy the stability requirements demanded of dielectrics for electronic constructional elements, e.g., the p,p'-disubstituted benzoic acid phenyl esters described in published German Specification No. 2,139,628 (U.S. application Ser. No. 277,502, filed Aug. 1, 1972, now U.S. Pat. No. 4,002,670) and the p,p'-disubstituted biphenyl derivatives described in published German Specification No. 2,356,085 (U.S. application Ser. No. 413,247, filed Nov. 6, 1973, now U.S. Pat. No. 3,947,375). In both of these classes of compounds, as in the case of other known series of compounds with a liquid crystalline mesophase, there are no individual compounds which form a liquid crystalline nematic mesophase in the temperature range of 10.degree. C. to 60.degree. C. Therefore, as a rule, there are employed mixtures of two or more compounds which are useable as liquid crystalline dielectrics, usually a mixture of at least one compound with low melting and clear point and one having a considerably higher melting and clear point. Normally, there is obtained a mixture whose melting point lies below that of the lower melting component thereof and whose clear point lies between the clear points of the components. Optimal dielectrics cannot be prepared in this manner because the component with the high melting and clear point almost always also imparts a high viscosity to the mixture. The switch times of the electro-optical constructional elements produced therewith are thereby prolonged in an undesirable manner.
It has now been found that the biphenyl esters of this invention are exceptionally suitable as components of liquid crystalline dielectrics, imparting thereto the requisite properties without imparting thereto an undesirably high viscosity.